Transmembrane pores (nanopores) have great potential as direct, electrical biosensors for a variety of analytes, such as polymers and small molecules. When a potential is applied across a nanopore, there is a change in the current flow when a molecule, such as a nucleotide or a polynucleotide, resides transiently in the barrel or channel of the nanopore for a certain period of time. Specific molecules, such as specific nucleotides and specific polynucleotides, give current changes of known signature and duration. Such current changes can be used to identify the nucleotide or polynucleotide present in the pore.